Some people are confused about what portable amps may or may not do, so I decided to post up a few thoughts. For starters, I am addressing the higher end portable amps and not the inexpensive cheap ones. Many of the cheap amps add unwanted hiss/hum, especially with overly sensitive IEMs or custom monitors with balanced armatures. Does that mean you have to spend a fortune on an amp, not necessarily? Just do your homework and don’t fall for the placebo effect as portable amps do not improve present poor sound quality. Keep in mind that many or most portable headphones do not necessarily need an amp (impedance of ≤ 60 ohms) and in my honest opinion a higher spend on headphones or iem's will deliver the most obvious improvement.

Some players may sacrifice the sound output (amp power) for size, battery life or other reasons, thus making portable amps a possible option. A few important things to keep in mind that affect the overall sound quality are the audio encoding* scheme that you use to encode your music, the quality of the original studio recording, the digital audio player and the headphones/IEMs you use. In my opinion the latter (headphones/IEMs) make the largest difference in improvement/performance. Either way, a good portable headphone amp should transform weaker players into more powerful portables that deliver a richer, fuller sound at lower listening levels. Even though I still believe; if your headphones/IEMs are bad or poor quality, you should first invest into a decent set of headphones, rather than spending money to amplify the bad headphones louder, thus "an amp is not a good fix for bad sound."

What can amp’s do:

Amplifies the signal for increased volume levels

Drives higher impedance headphones, amp can also help with low impedance iem’s that are too sensitive and produce hissing sound (possible impedance mismatching). A lower cost solution for this would be to use an impedance adapter, for example look here

"Possibly improve the stereo crosstalk (which might improve perceived soundstage/stereo separation) but you'd need a good amp for that, cheap ones like the FiiO E5 have enough crosstalk on their own," (thanks Martin)

Depending on which amp, it might add a new dimension or color the sound, I underline "might" as this is most likely placebo

Improves or increases the dynamic range from high to low, which can allow lower listening levels

Increases bass response, punch or thump, but this can also depend on the quality of phones to a high degree as well (see bad phones comment above). Also some amps may come with a bass boost option, which would deliver more bass but may introduce distortion

Bad or meager sounding headphones or DAP will not benefit or improve sound quality if you amp them

Placebo effect can be a factor, be careful and don't let yourself be fooled

*data rates of at least 192kbit/s (VBR) or higher and yes this may vary from listener to listener.